1. Field of the Invention
The present invention relates to an objective lens, an optical pickup device and an optical information recording or reproduction apparatus.
2. Description of Related Art
Hitherto an objective lens for performing recording of information on an optical disk and reproduction of information recorded on an optical disk by distributing much of the quantity of incident light beam into two diffracted lights having a different diffraction order from each other and converging a diffracted light having either of the diffraction orders onto an information recording surface of the optical disk, has been known (for example, see JP-Tokukaihei-9-179020A). There has been a problem that detection of a focus other than a focus used for recording or reproduction easily occurs in such an objective lens having a plurality of focuses when a focus pull-in operation is performed.
A bifocal objective lens disclosed in the above document distributes much of the quantity of incident light beam to two focuses by a diffraction structure formed on the lens surface to compatibly perform recording or reproduction on two types of optical disks having a same light source wavelength and different protective layer thicknesses from each other.
In general, a focus having a longer focal length is detected earlier in a plurality of focuses because an optical pickup device performs a focus pull-in operation by bringing an objective lens close to an optical disk gradually. Accordingly, when lens design is performed as the technique disclosed in the above document in which the distance along the optical axis between two focuses is not sufficiently long, there is a possibility of detecting the focus having the shorter focal length in focusing the focus having the longer focal length on the information recording surface of an optical disk.